A system for performing a therapeutic procedure is, for instance, a high dose rate (HDR) brachytherapy system. A HDR brachytherapy system comprises a brachytherapy catheter, which is introduced into the object to be treated like a prostate of a patient. Within the brachytherapy catheter a radioactive radiation source is moved to different dwell positions, at which the radioactive radiation source is located for a respective dwell time, wherein the object is treated by radioactive radiation emitted by the radioactive radiation source at the different dwell positions for the respective dwell times.
During the movement of the radioactive radiation source to the different dwell positions it can be difficult to know whether the respective dwell position within the brachytherapy catheter is within the patient or outside the patient. If a dwell position, at which the radioactive radiation source is located for a dwell time, is outside of the patient, the patient's outer skin and the environment, especially medical personnel performing the HDR brachytherapy procedure, may be affected by the radioactive radiation.